Hopefuls have a five-day period in which to declare their intention to stand in the 14 June poll.

More conservative than reformists have put themselves forward as candidates, reports say. Iran's Guardian Council - a body controlled by the Supreme Leader - decides who can stand.

Constitutionally, President Ahmadinejad cannot run for a third term.

The results of the last presidential elections, in 2009, were disputed by the opposition, triggering mass street protests.

Analysis

By Rana RahimpourBBC Persian Service

The doors of Iran's interior ministry have opened to those hoping to become Iran's next president.

To be eligible one must be male, a Shia Muslim, a political or religious figure, an Iranian national and believe in the core tenets of the Islamic Republic.

But this is just the beginning. After registration, the powerful 12-member Guardian Council, comprising six lawyers and six clerics, decides who qualifies to run.

The history of Islamic Republic shows that critics of the regime, even from within conservative circles, are routinely prevented from standing. None of the main pro-reform opposition figures or supporters of President Ahmadinejad has made it clear whether they intend to run or not.

If they do, they would typically wait until 11 May, the last day of the five-day window, to register.

The opposition says more than 80 of its supporters were killed over the following six months, though the government denies this.

Reformist candidates Mir Hossein Mousavi and Mehdi Karroubi were detained and remain under house arrest.

A reformist MP, Mostafa Kavakebian, who was disqualified by the Guardian Council in 2009, registered early on Tuesday.

Mr Rowhani's allies say he will withdraw his application if Mr Rafsanjani decides to run for president.

Mr Ahmadinejad is likely to endorse the expected candidacy of his chief-of-staff and close confidante, Esfandiar Rahim-Mashaei.

However, long-term tensions between Mr Ahmadinejad and the Supreme Leader Ayatollah Ali Khamenei have tainted Mr Rahim-Mashaei's reputation among some conservative elements.

In 2009, 475 hopefuls registered as candidates, but the Guardian Council, whose 12 members are either directly or indirectly appointed by the Supreme Leader Ayatollah Ali Khamenei, only gave its approval to four.